CN116075993A - Free-form collimating lens for angled facet laser devices - Google Patents

Abstract

An apparatus having a waveguide and a free-form collimating lens. The waveguide is characterized by a waveguide axis and a planar tip having a normal axis inclined at a tip angle greater than 0 degrees relative to the waveguide axis. The free-form collimating lens collimates light exiting the planar end of the waveguide into a collimated beam characterized by a beam direction parallel to the waveguide axis. The device suppresses reflection from the flat end of the waveguide from propagating back to the waveguide while providing a collimated light beam having a direction parallel to the axis of the waveguide.

Description

Free-form collimating lens for angled facet laser devices

Cross Reference to Related Applications

The present application claims priority and benefit from U.S. provisional patent application No. 63/066,070, filed 8/14/2020, the contents of which are hereby incorporated by reference in their entirety.

Background

Quantum cascade laser devices (QCLs) are typically composed of an elongated, approximately rectangular box of active semiconductor waveguide structure attached to a metal base that provides thermal and electrical contact. Divergent light emitted from the front facet of the waveguide of such devices is typically collimated using an aspherical high numerical aperture lens that is effective in correcting spherical aberration.

QCL device waveguides are most commonly fabricated with normally incident output facets. That is, the normal to the output facet is parallel to the long axis of the waveguide and the direction of light propagation. This is done for convenience and ease of manufacture because the light is emitted straight along the device axis so that it can be mounted right on the submount and collimated by a lens coaxially aligned with the waveguide, regardless of wavelength.

However, it has been demonstrated that introducing angles (typically 7-10 degrees) to the output facet of a quantum cascade laser waveguide can enhance its performance (maximum power, spatial mode quality) while reducing or eliminating the need for high complexity, high cost multilayer anti-reflective coatings.

Unfortunately, the introduction of angled facets adds complexity to a broader optical design. The angled facets refract light emitted from the waveguide at an angle relative to the long axis of the gain chip. In principle, the beam steering effect may be corrected by mounting the collimator lens at an angle with respect to the gain chip holder such that light enters the collimator lens along its central axis. This also requires the external cavity to rotate to accommodate the new beam direction. However, this solution introduces significant complexity to the mounting scheme of the gain chip and the collimator lens. In the case of wavelength dependence of the propagation angle from the waveguide, this solution also does not necessarily correct the refractive beam steering nor eliminate the non-rotationally symmetrical aberrations caused by the offset object plane distance.

Disclosure of Invention

The invention includes an apparatus having a waveguide and a free-form collimating lens. The waveguide is characterized by a waveguide axis and a planar tip having a normal axis inclined at a tip angle greater than 0 degrees relative to the waveguide axis. The free-form collimating lens collimates light exiting the planar end of the waveguide into a collimated beam characterized by a beam direction parallel to the waveguide axis.

In one aspect, the tip angle is greater than 7 degrees.

In another aspect, the device further comprises an optical amplifier that amplifies light reflected from the planar tip, the tip angle being selected such that the intensity of light reflected from the planar tip is insufficient to lasing in a system incorporating the device.

In another aspect, the optical amplifier includes a quantum cascade gain chip.

In another aspect, the optical amplifier includes a doped optical fiber.

In another aspect, the device further comprises an external cavity reflector that returns the collimated light beam to the free form collimating lens, the return light traveling in a direction parallel to the waveguide axis.

In another aspect, the external cavity has a wavelength selective filter.

In another aspect, the wavelength selective filter comprises a diffraction grating.

In another aspect, the free-form collimating lens includes a free-form concave surface proximate the planar extremity and a spherical convex surface distal the planar extremity.

In another aspect, the apparatus further comprises a lens holder that positions the free-form collimating lens relative to the quantum cascade gain chip such that an optical axis of the free-form lens is parallel to the waveguide axis.

Drawings

Fig.1 shows a typical QCL with an EC for tuning a laser.

Fig.2 shows a gain chip with facets cut at a slight angle.

Fig.3 shows a gain chip with a rotated collimating lens.

Fig.4 shows a collimating lens according to one embodiment of the present invention.

Fig.5 shows an example of a Main Oscillating Power Amplifier (MOPA).

Fig.6 illustrates a waveguide according to an embodiment of the present invention, which may be used in the amplifier shown in fig. 5.

Detailed Description

The manner in which the present invention provides its advantages can be more readily understood with reference to QCL that uses an External Cavity (EC) to tune the output wavelength. Referring now to fig.1, a typical QCL with an EC for tuning a laser is shown. The laser 40 includes a gain chip 41 mounted on a mount 42. Light emitted from the front facet 43 of the gain chip 41 is collimated by the lens 52 and reflected from the grating 46. The angle of the grating 46 relative to the beam from the gain chip 41 is selected to lock the laser in a particular mode. The angle is set by an actuator 45 which rotates the grating about an axis 53 which is selected such that the diffraction wavelength and the length of the cavity are maintained to provide the desired wavelength. The lens 47 expands the output beam to a desired size to provide output light for use by a measurement system using the laser 40 as its light source. The lens 52 expands the light leaving the front facet 43 of the gain chip 41.

The above description assumes that there is no reflection at facet 43. If facet 43 reflects light and is parallel to facet 48, both facets form a fixed length optical cavity that "competes" with the desired optical cavity provided by facet 48 and grating 46. To avoid this problem, the prior art external cavity lasers coat facet 43 with an anti-reflection coating that adds to the cost of the laser. Since these lasers are designed to be tunable over a large wavelength range, the cost of an antireflective coating that functions over the entire wavelength range can be significant.

In prior art systems, the gain chip is typically mounted to the mount by wire bonding to electrically connect the chip to the mount. The collimating lens is typically rigidly attached to a structure common to the chip carrier. The collimating lens holder typically requires multiple degrees of freedom to properly align the lens with respect to the gain chip.

A solution to this second cavity problem uses a gain chip in which facet 43 is not parallel to facet 48. Referring now to fig.2, a gain chip with facets cut at a slight angle is shown. The gain chip 11 has facets 12 cut at an angle between 7 ° and 10 ° with respect to the facets 13. Since the two ends of the gain chip are no longer parallel, the light reflected from the ends cannot form a resonant cavity. The angled surfaces cause refraction of light exiting facet 12 relative to the direction that light would travel through a conventional vertical facet. Two problems are encountered if the collimator lens 14 is kept in the same position as a corresponding collimator lens used with non-slanted facets.

To generalize the following discussion to other optical systems, the angle of the end of the waveguide will be specified by the angle between the plane of the end and the optical guide axis. In this nomenclature, the angle between the slanted tip and the waveguide axis is 80 degrees to 83 degrees.

First, the light leaving the facet 12 is not a point source, but extends over a small area on the facet 12. Thus, a portion of the light will now be emitted at a point different from the focal point of the collimator lens 14. This results in impaired collimation of the light.

Second, the lower portion of the collimating lens 14 is not optimally used because the beam from the angled surface does illuminate the lower portion of the lens as much as possible.

One way to reduce these problems involves rotating the collimating lens 14 to compensate for the variation in emission angle introduced by the slanted facets. Referring now to fig.3, a gain chip with a rotated collimating lens is shown. In the configuration shown in fig.3, the collimator lens 14 has been rotated such that the optical axis 15 of the collimator lens 14 is perpendicular to the surface of the facet 12 and is positioned at the center of the area of emitted light. While this solution partially compensates for the optical aberrations discussed above, it requires a complex mounting arrangement for the collimator lens 14. Furthermore, the axis of the collimated beam is no longer collinear with the axis of the waveguide of the gain chip 13. This requires changing the mounting system of the diffraction grating, which also increases the cost of the laser.

The lasers of the present disclosure overcome these problems by compensating for the effects of oblique facets using a collimating lens that may be placed at the home position. As used in this disclosure, the term "collimated" describes a beam that can propagate laboratory-scale distances (a few centimeters to a few meters) without significant changes in size (beam divergence reduced to milliradian levels). For mid-infrared (wavelength of about 4 to 12 microns) beams, the beam waist must be several millimeters. In one exemplary embodiment, the beam waist is 5 millimeters. In another exemplary embodiment, the beam waist is 7 millimeters. Similarly, in one exemplary embodiment, the focal length of the collimating lens is 2 millimeters to 5 millimeters.

Referring now to fig.4, a collimating lens in accordance with one embodiment of the present invention is shown. The gain chip 11 comprises angled facets 12 that emit light at a position centered on the optical axis of the waveguide in the gain chip 11. Light rays leaving facet 12 are collimated by a lens, which will be referred to as a "free-form lens". The free-form lens 60 has a convex spherical surface 61 and a concave surface 62 having a shape calculated to collimate light rays emitted from the facet 12. This inner surface will be referred to as the free-form surface.

Free-form lenses of the type shown in fig.4 are known in the art and will therefore not be discussed in detail herein. For the purposes of this disclosure, it is sufficient to note that the free-form surface is defined by a function having a plurality of variable parameters determined such that the shape of the surface, together with the lens material and the other surface of the lens, provides the desired collimation. In one aspect, the freeform surface is represented as a three-dimensional polynomial having a plurality of freeform parameters determined such that light rays exiting facet 12 and passing through the surface area will diffract into light rays exiting lens 60 in a direction parallel to optical axis 64. The function describing each small surface area is further constrained such that the entire surface constructed from the collection of surface areas is a continuous and smooth surface. The respective polynomial surfaces are preferably at least quadratic or higher order. Furthermore, the parameters and points at which the surface areas join each other are selected such that the first derivative of the entire surface is continuous at the point at which the two surface areas join.

The parameters of lens 60 are also selected so that lens 60 and gain chip carrier 68 may be secured to a common surface 67. In this arrangement, the optical axis 64 is collinear with the axis 65 of the waveguide of the gain chip 11. Thus, the existing geometry of the external cavity laser can be maintained while correcting for artifacts introduced by the angled facets 12 and thus significantly reducing the need for an anti-reflective coating on the facets 12.

The above embodiments use a grating for the wavelength selective filter and a reflector for the external cavity. However, the collimation system of the present disclosure may be used with any external cavity quantum cascade laser to maintain the linear geometry of the laser cavity while correcting for distortion introduced by the slanted facet edges on the gain chip. Furthermore, such a collimation system may be used with other wavelength selective filters within the external laser cavity.

The above embodiments relate to lasers; however, the system of the present disclosure may be advantageously used in other optical systems, wherein light reflected from the exit facet of the waveguide may cause the system to laser, as the exit facet provides one surface of the resonant cavity for light of the wavelength in question. Referring now to fig.5, an example of MOPA is shown. MOPA80 uses seed laser 81 to generate an optical signal that is coupled to optical amplifier 82 through coupler 83. The power supply for the optical amplifier 82 has been omitted from the drawing for simplicity of the drawing. The output of the optical amplifier 82 is provided to an output optical fiber 84. The output fiber may be part of an optical amplifier 82. For example, the optical amplifier 82 may be a doped fiber that is pumped to provide amplification. Doped fibers for amplifying optical signals are known in the art and will therefore not be discussed in detail herein. In this case, the output fiber 84 may be the end of a doped fiber. The output of the output fiber 84 is typically expanded into a collimated beam 87 by a collimating lens 86. The collimating lens 86 is rigidly connected to the output optical fiber 84 by a mounting structure 88, similar to the mounting structure discussed in the previous embodiments of the laser.

One problem with the arrangement shown in fig.5 arises from reflection at surface 85. Typically, surface 85 is perpendicular to the axis of output fiber 84. If the reflectivity at surface 85 is sufficient, a laser cavity is formed by surface 85 and the reflective surfaces in seed laser 81 or coupler 83. Thus, the optical amplifier 82 may become a separate laser having a different spectral pattern from that of the seed laser 81. Thus, surface 85 is typically coated with an anti-reflective coating, which increases the cost of MOPA 80.

Referring now to fig.6, a waveguide is shown in place that may be used for waveguide 84 shown in fig. 5. Waveguide 95 has an end surface 96 cut at an angle other than 90 degrees relative to optical axis 97. The angle of the end surface 96 relative to the axis 97 is selected such that reflections from the surface 96 are not directed back towards the waveguide 95 and thus avoid the lasing problems discussed above. The freeform lens 90 provides collimation of the output light from the waveguide 95 in the direction of the axis 97. In this example, the optical axis 93 of the collimated beam coincides with the optical axis of the waveguide 95. The waveguide 95 is fixed on a mount 98 that is rigidly positioned relative to the free-form lens via attachment to the common surface 94.

Exemplary embodiments

Embodiment 1 an apparatus comprising: a waveguide characterized by a waveguide axis and a planar tip having a normal axis inclined at a tip angle greater than 0 degrees relative to the waveguide axis; and a free-form collimating lens that collimates light exiting the planar end of the waveguide into a collimated beam characterized by a beam direction parallel to the waveguide axis.

Embodiment 2. The device of embodiment 1, wherein the tip angle is greater than 7 degrees.

Embodiment 3 the device of embodiment 1 or 2 further comprising an optical amplifier that amplifies light reflected from the flat tip, the tip angle preventing lasing in a system incorporating the device.

Embodiment 4. The device of embodiment 3, wherein the optical amplifier comprises a quantum cascade gain chip.

Embodiment 5. The device of embodiment 3, wherein the optical amplifier comprises a doped optical fiber.

Embodiment 6 the device of any one of embodiments 1 to 4, further comprising an external cavity reflector that returns a collimated light beam to the free-form collimating lens, the returned light traveling in a direction parallel to the waveguide axis.

Embodiment 7 the device of embodiment 6 comprising a cavity external to the quantum cascade gain chip, wherein the cavity comprises a wavelength selective filter.

Embodiment 8. The device of embodiment 7, wherein the wavelength selective filter comprises a diffraction grating.

Embodiment 9 the device of any one of embodiments 1-8, wherein the free-form collimating lens comprises a free-form concave surface proximate the planar extremity and a spherical convex surface distal the planar extremity.

Embodiment 10 the device of any one of embodiments 1-9, further comprising a lens holder that positions the free-form collimating lens relative to the quantum cascade gain chip to maintain the beam direction parallel to the waveguide axis.

The above embodiments of the invention have been provided to illustrate various aspects of the invention. It is to be understood, however, that different aspects of the invention, which are shown in different particular embodiments, may be combined to provide further embodiments of the invention. In addition, various modifications of the present invention will become apparent from the foregoing description and accompanying drawings. Accordingly, the invention is limited only by the scope of the appended claims.

Claims (10)

1. An apparatus, comprising:

a waveguide characterized by a waveguide axis and a planar tip having a normal axis inclined at a tip angle greater than 0 degrees relative to the waveguide axis; and

a free-form collimating lens that collimates light exiting the planar end of the waveguide into a collimated beam characterized by a beam direction parallel to the waveguide axis.

2. The device of claim 1, wherein the tip angle is greater than 7 degrees.

3. The device of claim 1, further comprising an optical amplifier that amplifies light reflected from the flat tip, the tip angle preventing lasing in a system incorporating the device.

4. The apparatus of claim 3, wherein the optical amplifier comprises a quantum cascade gain chip.

5. The apparatus of claim 3, wherein the optical amplifier comprises a doped optical fiber.

6. The device of claim 4, further comprising an external cavity reflector that returns a collimated light beam to the free-form collimating lens, the returned light traveling in a direction parallel to the waveguide axis.

7. The device of claim 6, comprising a cavity external to the quantum cascade gain chip, wherein the cavity comprises a wavelength selective filter.

8. The apparatus of claim 7, wherein the wavelength selective filter comprises a diffraction grating.

9. The device of claim 1, wherein the free-form collimating lens comprises a free-form concave surface proximate the planar extremity and a spherical convex surface distal the planar extremity.

10. The device of claim 4, further comprising a lens mount that positions the free-form collimating lens relative to the quantum cascade gain chip to maintain the beam direction parallel to the waveguide axis.

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